(1) Field of the Invention
This invention relates to a switching power supply unit with a battery backup function by a secondary battery for obtaining a predetermined direct-current power supply voltage at the time of, for example, a stoppage in commercial power supply and, more particularly, to a switching power supply unit which can stably continue the supply of power to units, such as an information processing unit, connected thereto even in case of a stoppage in alternating-current input from commercial power supply.
(2) Description of the Related Art
A power failure, such as a stoppage in power supply, or a fault may occur at any time in a power supply unit which uses input from commercial power supply. Such a situation can not be expected easily. If a backup of data in an information processing unit, such as a personal computer system, is not stored in a nonvolatile storage medium, then it will be lost when such a power failure occurs. If a power failure or a fault continues for several tens of milliseconds or more, a personal computer may lose data.
One of solutions to this problem is to use an uninterruptible power supply (UPS). If a UPS is used, the supply of power to a personal computer, a server, or a mainframe can be continued when the supply of power from commercial power supply is disturbed or stopped. That is to say, a computer system can maintain its normal operation state without an interrupt to a user. However, there is a limit to time for which a UPS can supply power to a computer system.
Therefore, if a power failure continues for time longer than time for which a UPS can supply power (that is to say, when electric charges stored in a UPS run out), the supply of power to a computer system is stopped and data and an application which are now used or displayed will be damaged. Moreover, a UPS is expensive and usually commercial users use it. Furthermore, the longer a UPS can supply power, the more expensive it becomes. Therefore, most of small-scale businesses and domestic users cannot buy a UPS, so the possibility that they will lose data at the time of a power failure or a fault becomes greater.
For example, Japanese Patent Laid-Open Publication No. 2000-14043 therefore discloses a built-in uninterruptible power supply for computers which operate by commercial power supply. This type of uninterruptible power supply can be integrated into a personal computer system having a power management mechanism, such as suspend/resume. If an alternating-current power failure has occurred, a reliable low-priced power supply backup system should be used to hold data.
A UPS is connected to the outside of an information processing unit and is used. This needs unit space. Moreover, a UPS is expensive. Conventionally, the use of a stabilized DC power supply with a backup function therefore has been considered. For example, Japanese Patent Laid-Open Publication No. 2002-27683 discloses an electronic unit with a backup power supply the system structure of which is simplified by making it unnecessary to use an uninterruptible power supply.
With conventional backup power supplies, however, a switch for prohibiting discharging is contained in a battery pack and one terminal is used for charging and discharging. Therefore, it is difficult to restrain a leakage current from flowing from a battery to a converter section. Moreover, the size of a switching power supply unit with a backup power supply will increase if a battery is contained in a unit enclosure. Therefore, many switching power supply units with a backup power supply do not contain a battery. In addition, the structure in which a switching power supply circuit is mounted in, for example, a drive bay in the enclosure of an information processing unit has been adopted, so extra mounting space and wirings are necessary.
A dedicated bay (slot) for mounting a battery is also necessary, especially if a switching power supply circuit is mounted in a bay. As a result, the entire battery pack becomes expensive.
Furthermore, unlike a server or a personal computer used in a room equipped with an air conditioner, a secondary battery used for factory automation (FA) has following problems. If a secondary battery is used at an extremely high ambient temperature, the life will shorten. If a secondary battery is used at an extremely low ambient temperature, its internal impedance will increase and sufficient power will not be obtained.